... and SA is corrected to a very low level. Later this week or perhaps next week, I will take some images of a real star and compare to them to my indoor test.
I was struck by how much better the images were then a previous 110 F/5.6 that I had owned...
As I built the lens of this scope, please allow me to make some comments regarding the indoors test:
The optics are corrected for objects in infinite distance. The most reliable test is a real star.
Before testing, care must be taken to allow time for the optics to cool/thermalize as perfectly as possible. If the edge of the glass is coolder, spherical undercorrection will be visible. It will be gone after the lens is properly cooled.
At green color (532nm wavelength) you should NOT see more than about lambda/50 spherical error (3rd, 5th and 7th order sphericals combined). This is the normal level of spherical correction we aim, and we release lenses that are "worse" than this only when we have a very specific reason (e.g. Strehl is already very high and further figuring would risk making the lens actually worse).
On the other hand, due to the (natural) spherochromatism, the spherical correction slightly changes with wavelength. So, testing in red light will show a small spherical undercorrection and using blue light will show the lens spherically overcorrected. This is natural.
To see the real spherical correction with minimal "distortion" using an artificial star, you should use a narrow band green light filter (or use a laser based artificial star radiating monochromatic 532nm light) and put the artificial star as far away from the lens, as possible. The VIRTUAL spherical undercorrection (resulting from the finite object distance) in different cases:
- 5m (16.4 feet) distance results in lambda/4 (!!!) spherical undercorrection
- 10m (33 feet) distance results in lambda/12 spherical undercorrection
- 15m (49 feet) distance results in lambda/15 spherical undercorrection.
These values are valid for our 92mm APO. Larger lenses need even larger distances.
The non-concentric light distribution in the defocused images might be the result of uneven amounts of light reaching different parts of the lens. This looks like coma but the real problem might be the uneven light radiation pattern of the artificial star. Using a real star (where the light flux is surely constant over the whole optical surface) this should NOT be visible.
Anyway, I really hope the scope will bring you many happy observing hours... :-)
Edited by gyulaipal, 12 October 2016 - 11:35 AM.